Since my game till just recently was a war campaign we had several big battles. One part of it was the Red Hand of Doom Drellin's Ferry retreat (or taken from that) but in which I actually ran the enemy units while the pcs commanded two companies of militia in a fighting retreat.
We also had a mass combat against Frost Giant mercenaries and their minions (winter wolves and wyvern riding hobgoblin shamans) with the pcs leading their followers and cohorts.
The big climax was against a lich and his horde of undead, nearly 30 ghouls and ghasts, a Shadow Dragon and the lich himself. The party nearly got wiped out but won in the end.
I suggest for units of humanoids/undead or whatever that you have them fight as units and give them feats that are appropriate--swarm is good for chaotic or mindless creatures for instance. Formation fighting also works. The tricky npcs/monsters to work with are those that have a variety of special powers, feats or spells--you really need to plan that in advance with a couple of contingency plans for general circumstances. It is a lot of work but also really fun.